<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <title>像素</title>
    <style>
        .atguigu1 {
            width: 1cm;
            height: 1cm;
            background-color: red;
        }
        .atguigu2 {
            width: 1mm;
            height: 1mm;
            background-color: green;
        }
        .atguigu3 {
            width: 100px;
            height: 100px;
            background-color: blue;
        }
    </style>
</head>
<body>
    <div class="atguigu1"></div>
    <br>
    <div class="atguigu2"></div>
    <br>
    <div class="atguigu3"></div>
</body>
</html>